Bicycle Mechanics - Used fork compatibility

I'm looking for a used fork for a late-70s Raleigh Grand Prix. New forks won't do because they have less rake and I already have a bit of wheel-toe overlap.

Fortunately, there is a bike shop in my area that has lots of used parts from old bikes, so I should be able to find something. However, I want to go in well-prepared to make sure I get a compatible fork (I'm planning to do the work myself so I don't want to walk away with a fork only to find days later that it won't fit).

What measurements do I need to take to make sure that my new fork will be compatible with the frame?

Bring your used fork with you. Especially note the fork crown race diameter (this you'll need a caliper to measure). You'lll want to match the same length fork, and same length of threads (i'm assuming a threaded fork).

Especially note the fork crown race diameter (this you'll need a caliper to measure).

Do you mean to measure the diameter of the fork at the point where the crown race attaches? Or the diameter of the race itself?

If you want to reuse your headset, match both. Assuming they were already a good fit to begin with.

Ah, ok. Actually the reason I'm replacing the fork is that the old one has non-standard threads, which is preventing me from replacing my worn-out headset. (I have tried an old, used Raleigh headset but it didn't work very well). So no, I'm not planning to reuse the headset.

Make sure that the new fork steere is compatable in length with the stack height of your chosen headset.
You need the difference between your head tube and the new steerer length. If your new steerer is to long for the stack height you can add a spacer to the stack. If it is too short you cant use that HS.
If the rake of the new fork is a bit different the handling may change but may still be acceptable. My bike got a bit flighty with a newer (used) fork but you get used to it.